In modern production machinery, such as machine tools, it is necessary to have precise position detectors for accurately measuring and producing large parts. Often, such machinery are large and bulky, and require the use of long and precise position detectors which are difficult to manufacture and mount accurately. These position detectors are usually mounted to two members of the machine tool and provide information on the displacement between the two members. The present invention relates to improvements to a position detector used for measuring the relative displacement between two members.
Various arrangements have been used in the art for providing positional information, for example, by providing an electrical signal which varies with displacement. Such an arrangement is disclosed in GB 1513567, wherein a first member carrying a row of balls moves relative to a second member carrying electromagnetic induction coils and pick-up coils. A magnetic field is induced along the line of contact of the balls by the induction coils and the signal output from the pick-up coils depends on the position of the balls. Relative displacement between the first and second members results in movement of the balls past the pick-up coils, thus providing a signal varying with displacement. In such an arrangement, to calculate relative displacement, the number of periods of the signal must be counted.
Although the device disclosed in GB 1513567 has been commercially successful, it has limited resolution, and is incapable of automatic calibration or self-diagnosis. In addition, this prior art position detector generates non-standard signals which require equipment designed by the original manufacturer of the detectors, commonly known as OEM, to decode the signals, and thus provide the positional information.
Prior art devices, such as that in GB 1513567, are manually calibrated by a skilled operator. Generally, following manufacture and assembly of the device and prior to use in service, the accuracy of the device is compared against a standard device and adjusted to minimise deviations. This adjustment is done using manual potentiometers within the device.
Once in service, re-calibration is awkward. In addition, the devices are not able to adjust themselves to take account of fluctuations in signalling due to, for example, changes in ambient temperature, operational frequency, or even in the properties of the components of the device over time. Although these fluctuations may be relatively small, they do affect the accuracy of the device. A further drawback of prior art devices is that they provide no immediate indication that the device requires re-calibration, which is particularly important for small but significant deviations.
The frequency of operation determines the resolution of the prior art devices. These devices are generally operated at a relatively low frequency of 1 kHz (1000 cycles per second). Although the devices can be operated at higher frequencies and could provide higher resolutions, they would require higher frequency clocks which are more expensive. In addition, higher operational frequencies lead to a change in the magnetic properties of the system, which in turn would, without correction, lead to increased errors in measurement of position.
Furthermore, the interpolation of the signalling to provide positional information can only occur once a cycle in the prior art devices. This is because the prior art devices operate by comparing the phase shift between the drive and return signals, which is most conveniently done by comparing the zero amplitude in the signalling. As the interpolation can only occur once a cycle, for the size of balls and frequency of operation which are commonly used, the prior art devices are limited to approximately 2.5 μm resolution. However, improvements in the speed of operation of production machinery means that, in certain cases, this resolution is not adequate. Accurate resolution of the order of 1 μm and less is required.